Rollup merge of #97482 - RalfJung:ptr-invalid, r=thomcc

ptr::invalid is not equivalent to a int2ptr cast

I just realized I forgot to update these docs when adding `from_exposed_addr`.
Right now the docs say `invalid` and `from_exposed_addr` are both equivalent to a cast, and that is clearly not what we want.

Cc ``@Gankra``